Chemical pesticides have become a mainstay of modern agriculture, despite red flags that they’re slowly destroying ecosystems. (Technically, pesticides are designed to kill insects, while herbicides are used to kill weeds or substances like bacteria or fungi, but when discussing them, the U.S. EPA lumps them all together as “pesticides.”¹)

Part of what makes assessing the health and environmental risks of pesticides so difficult is that many of the risks remain unknown, and those that are known can be difficult to quantify, no matter what they’re called.

So, researchers like Robert Brucker, who heads up a lab in the Microbial Sciences Initiative at the Rowland Institute of Harvard, are invaluable. Brucker and colleagues are looking into the hidden risks of pesticides — silent, insidious changes that are occurring before our eyes yet often under the radar, such that standardized risk assessments do not consider them, but should.

In research published in Cell Host & Microbe, the researchers looked into the effects of atrazine, an herbicide that’s banned in the European Union but widely used in the U.S., on Nasonia vitripennis, a wasp. According to the study, “atrazine is the second-most-sold pesticide globally.”²

The study revealed that exposure to low doses of atrazine leads to changes in the gut microbiota of the wasps, increasing their resistance to pesticides. What’s more, the changes were transferred to subsequent generations. “The big surprise,” Brucker said in a news release, “was that one exposure, even at a nontoxic level, was enough to cause a heritable change in the microbial community.”³

Atrazine Changes Gut Microbes, Increases Pesticide Resistance

Pesticides are an example of one of the greatest exposure risks to xenobiotics — chemical compounds found in a living organism that are not normally produced or consumed by the organism in question. When consumed, xenobiotics are either absorbed directly by the gut or metabolized by gut microbiota.

It’s often said that atrazine is a “safe” pesticide because animals lack the ability to metabolize it, but it’s known that bacteria in soil and water can metabolize the chemical.⁴

“We chose atrazine because it’s widely used for corn crops, and is considered to be safe to most animals: Up to 3 [parts per billion, or ppb] is allowable in drinking water,” Brucker stated. “One of the first things we did was evaluate whether it was even toxic to our wasps — we had no reason to think it would be [at lower doses].”⁵

For the study, researchers exposed wasps to atrazine at concentrations of 300 ppb, an amount meant to simulate the exposure level that would occur among pollinators visiting a field that was recently sprayed. Changes in gut microbiota occurred, which increased the insects’ tolerance to pesticides, including not only atrazine but also glyphosate — a chemical to which the wasps had not been exposed.

The microbial changes in the wasps’ guts persisted across successive generations, meaning that the wasps’ offspring were affected even though they weren’t exposed to atrazine.

“Pesticide resistance is something that agriculture scientists are continually looking to avoid, so this finding is important,” Brucker continued. “When the wasps were only exposed to only 30 ppb of atrazine, the effect over 36 generations was increased resistance.”⁶

Wild Honeybees Also Affected by Atrazine

The researchers then screened wild honeybees from Brucker’s Ohio family farm, which live in an area with cornfields that had been sprayed with atrazine. Bacterial genes capable of degrading atrazine and, probably, other xenobiotics were found — genes that were “nearly identical” to genes found in the laboratory wasps.⁷ Writing in Cell Host & Microbe, the researchers concluded:⁸

“The rare gut bacteria Serratia marcescens and Pseudomonas protegens contributed to atrazine metabolism. Both of these bacteria contain genes that are linked to atrazine degradation and were sufficient to confer resistance in experimental wasp populations.

Thus, pesticide exposure causes functional, inherited changes in the microbiome that should be considered when assessing xenobiotic exposure and as potential countermeasures to toxicity.”

Notably, despite increasing attention to the extensive role the gut microbiome plays in health, host-microbiome interaction is not part of standard biorisk assessments for pesticides, though Brucker believes it should be. He stated in a Harvard news release:

“Everyone feels strongly about protecting our pollinator species, so we may need to be mindful of our uses of xenobiotics in crop management.

We need to understand multigenerational exposure better, and make host–microbiome interaction part of biorisk assessment in the future, especially in light of increasing xenobiotic exposure to humans, plants, animals, fungi, and bacteria across the globe.”⁹

Moving toward regenerative agriculture is the answer to reducing the toxic reliance on pesticides that is harming the environment and pollinators along with it. However, in the meantime Brucker and colleagues are even looking to develop probiotics for wasps that could help to stem some of the risks caused by multiple exposures to pesticides.